The present invention relates generally to thermal energy management systems within electron beam generating devices, and more particularly, to an assembly for cooling an x-ray tube window.
There is a continuous effort to increase x-ray imaging system scanning capabilities. This is especially true in computed tomography (CT) imaging systems. Customers desire the ability to perform longer scans at high power levels. The increase in scan time at high power levels allows physicians to gather CT images and constructions in a matter of seconds rather than several minutes as with previous CT imaging systems. Although the increase in imaging speed provides improved imaging capability, it causes new constraints and requirements for the functionality of the CT imaging systems.
CT imaging systems include a gantry that rotates at various speeds in order to create a 360xc2x0 image. The gantry contains a x-ray tube, which composes a large portion of the rotating gantry mass. The CT tube generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential is created across the vacuum gap allowing electrons to be emitted, in the form of an electron beam, from the cathode to a target within the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target, whereby they are abruptly slowed down to emit x-rays. The high voltage potential produces a large amount of heat within the x-ray tube, especially within the anode.
Typically, a small portion of energy within the electron beam is converted into x-rays; the remaining electron beam energy is converted into thermal energy within the anode. The thermal energy is radiated to other components within a vacuum vessel of the x-ray tube, and is removed from the vacuum vessel via a cooling fluid circulating over an exterior surface of the vacuum vessel. Additionally, electrons within the electron beam are back scattered from the anode and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result, the x-ray tube components are subject to high thermal stresses decreasing component life and reliability of the x-ray tube.
The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bi-polar configuration. High temperatures at an interface between the vacuum vessel and a transmissive window in the casing cause the cooling fluid to boil, which may degrade the performance of the cooling fluid. Bubbles may form within the fluid and cause high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images.
Prior art cooling methods have primarily relied on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. The coolant fluid is often a special fluid for use within the vacuum vessel, as opposed to the cooling fluid that circulates about the external surface of the vacuum vessel. Other methods have been proposed to electromagnetically deflect backs-scattered electrons so that they do not impinge on the x-ray window. These approaches, however, do not provide for significant levels of energy storage and dissipation. Due to inherent poor efficiency of x-ray production and desire for increased x-ray flux, heat load is increased that must be dissipated. As power of x-ray tubes continues to increase, heat transfer rate to the coolant can exceed heat flux absorbing capabilities of the coolant.
A thermal energy storage device or electron collector, coupled to an x-ray window, has been used to collect back scattered electrons between the cathode and the anode. In using this device the collector and window need to be properly cooled to prevent high temperature and thermal stresses, which can damage the window and joints between the window and collector. High temperature on the window and collector can induce boiling of coolant. Bubbles from boiling coolant obscure the window and thereby compromise image quality. Further boiling of the coolant results in chemical breakdown of the coolant and sludge formation on the window, which also results, in poor image quality.
A heat exchange chamber has been coupled to the electron collector, including a cooling channel, which allows coolant to flow in the channel across each of four walls of the electron collector. Although, the heat exchange chamber aids in cooling the electron collector, it is difficult to effectively manufacture due to its complexity and large number of seams, which each need to be properly sealed. Also, the heat exchange chamber is minimally effective in cooling of and preventing deposits from forming on the x-ray tube window. For further description of the electron collector or of the heat exchange chamber see U.S. Pat. No. 6,215,852 B1.
It Would therefore be desirable to provide an apparatus and method of cooling an x-ray tube window, thus an x-ray tube, that allows for increased scanning speed and power, is relatively easy to manufacture, and minimizes blurring and artifacts in a reconstructed image.
The present invention provides an assembly for cooling an x-ray tube window. An x-ray tube window cooling assembly for an x-ray tube is provided. The cooling assembly includes an electron collector body coupled to an x-ray tube window and having a first coolant circuit. The coolant circuit includes a coolant inlet and a coolant outlet. The coolant outlet directs coolant at an x-ray tube window surface to impinge upon and cool the x-ray tube window. The coolant is reflected off the reflection surface as to impinge upon and cool the x-ray tube window. A method of operating the x-ray tube is also provided.
The present invention has several advantages over existing x-ray tube cooling systems. One of several advantages of the present invention is that it provides an apparatus for directing coolant at an x-ray tube window. By directing coolant at the x-ray tube window the window is efficiently cooled, deposit formation on the window is minimized, and deposits are washed away as soon as they are formed, thus minimizing blurring and artifacts in a reconstructed image.
Another advantage of the present invention is that it provides a cooling mechanism or fin pocket, which effectively removes thermal energy from the coolant. The fin pocket is located on a coolant side of the electron collector body, providing relative ease in manufacturing of the present invention.
Furthermore, the present invention provides additional x-ray tube window cooling via an auxiliary cooling circuit, further allowing for Increased scanning speed and operating power, while being able to effectively cool the x-ray tube window.
The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures.